Enteric virus infections cause significant morbidity and mortality worldwide. Of these, human noroviruses (HuNoV) are severely understudied as they do not grow in small animals or readily available cell culture models and no directed disease prevention and control strategies exist. Answers to fundamental questions as to how these viruses enter cells remain unknown. For noroviruses, the initial events in the viral life cycle of receptor binding and entry are key in determining which cell types and tissues can be infected and ultimately the outcome of an infection. However, the knowledge of norovirus entry mechanisms is pivotal in designing therapies or strategies to combat disease or spread. Murine norovirus (MNV) is closely related to HuNoV and shares many biological and molecular characteristics. In contrast to HuNoV, MNV can be reverse engineered, replicates in tissue culture and provides a small animal model system. Therefore, the MNV model provides a tractable system to elucidate the viral and cellular determinants of norovirus entry for the first time. The current proposal will test the central hypothesis that specific residues in the P domain of the MNV-1 capsid interact with moieties on the host cell surface and this interaction leads to virus infection of cells. The following specific aims are designed to test this hypothesis: 1) Define the role of sialic acid during MNV-1 attachment;2) Identify viral determinants of MNV-1 entry;3) Define cellular mechanisms of MNV-1 entry into murine macrophages and dendritic cells. The methods used to address these aims include targeted mutagenesis of the viral capsid protein followed by infection studies in vitro and in vivo, fluorescence microscopy as well as molecular and cell biological approaches. Successful completion of these aims may reveal novel targets for antiviral drug design. It will advance our understanding of the norovirus life cycle and how viruses hijack the cellular machinery to infect macrophages and dendritic cells, particularly those of the GI tract. As such these studies will advance the fields of virology, cell biology and immunology. PUBLIC HEALTH RELEVANCE: Norovirus infections in humans cause frequent outbreaks of "stomach-flu", but no vaccines or treatments are available for use. For noroviruses, the first steps in the life cycle are critical in determining which cells are infected. Therefore, our goal is to improve our understanding of the role of both the host and the virus in these early events during norovirus infection in the hopes of identifying new targets for antiviral drug development.